1. Field of the Invention
This invention relates to circuit breakers having a thermal-magnetic trip unit, and more particularly to such circuit breakers having a heater transformer for the bimetal providing the thermal trip capability.
2. Background Information
The thermal-magnetic trip unit of a common type of circuit breaker has a heater which heats the bimetal providing the thermal trip response in proportion to the current drawn by the load. This heater comprises a transformer having a laminated core forming a loop interrupted by a gap. The bus strap forming a conductor between the load terminal and the separable contacts of the circuit breaker passes through the transformer core to serve as a one turn primary winding for the heater transformer and an electrically conductive sleeve forms a one turn secondary winding. The bus strap induces a magnetic flux in the laminated core proportional to the current through the closed contacts of the circuit breaker. This magnetic flux in turn induces a secondary current which circulates in the electrically conductive sleeve and generates heat. The bimetal is secured at one end to the electrically conductive sleeve and is thus heated thereby. The free end of the heated bimetal is deflected to engage and release a latch mechanism which trips the circuit breaker open. The secondary current and the heat applied to the bimetal are directly related to the load current and inversely related to the gap in the laminated core of the transformer. Therefore, by appropriate selection of the initial gap between the cold bimetal and the latch mechanism, and the gap in the core of the transformer, the current/time characteristic of the thermal trip can be established.
In one such circuit breaker of this type, the laminated core is U-shaped and a steel cross member is secured across the ends of the U by screws extending into the laminations of the U-shaped member. Brass spacers between the ends of the U-shaped core and the cross member form the gap in the magnetic circuit which is adjusted by selection of the thickness of the brass spacers. This arrangement requires drilling and tapping holes into the ends of the laminations of the U-shaped core. In addition, the electrically conductive sleeve forming a secondary of the heater transformer is captured on the cross member.
In another type of bimetal heater, the laminated core is C-shaped and an iron bar partially fills the gap between the confronting legs of the C-shaped core with the length of the iron piece selected to provide the proper air gap, and therefore, selected reluctance for the magnetic circuit. The electrically conductive sleeve forming a secondary is long enough to bridge the total gap between the confronting legs of the C-shaped core, to therefore retain the sleeve permanently in place and to capture the iron bar. Electrical insulation is provided between the electrically conductive sleeve and the core and the iron bar.
In these thermal magnetic trip units, a steel pole piece is secured to the laminated core to concentrate flux in the direction of an armature spaced from the pole piece. Instantaneous load currents of a predetermined magnitude generate sufficient flux to attract the armature to the pole piece thereby also unlatching the trip mechanism to trip the circuit breaker.
It is desirable in many instances, to provide interchangeable trip units in a molded case circuit breaker so that the same frame can be used for installations requiring different rated currents. One means for providing a different current setting for the thermal trip, is to adjust the gap in the magnetic circuit of the heater transformer. In the above-described circuit breakers, this is not easily accomplished. It would be desirable to be able to have interchangeable trip units which could be easily inserted in the molded casing of such a circuit breaker to provide the desired range of current ratings.
There is a need therefore for improved circuit breaker having a thermal-magnetic trip with an electro-magnetic heater.
There is a more particular need for such an improved circuit breaker in which the thermal trip characteristic can be easily and reliably adjusted. There is a more specific need for such an improved circuit breaker in which the thermal trip characteristic can be adjusted by changing the gap in the magnetic core of heater transformer. There is a further need for such an improved circuit breaker in which different trip units may be interchangeably inserted and withdrawn with each providing a different gap in the magnetic circuit to provide a range of current ratings for the circuit breaker.